Aqueous coating agent, process for its preparation, and its use for coating cans, based on acrylic monomers, epoxy-polyesters and methylol functional curing agents

Abstract

The invention relates to an aqueous coating agent, obtained from an epoxy resin, ethylenically unsaturated monomers, some of which contain carboxyl groups, a peroxide initiator in a proportion of at least 2% by weight, relative to the total weight of the monomers, a crosslinking agent, a neutralizing agent, organic solvents, and, if appropriate, further conventional additives, such as plasticizers, stabilizers, wetting agents, dispersion auxiliaries, catalysts and pigments. The coating agent is based on a binder a) which is obtainable from A) 20 to 80% by weight of an epoxy resin having an average of more than one epoxy group per molecule and having an average molecular weight of at least 500, B) 1 to 60% by weight of polyester polycarboxylic acids having an average molecular weight of 500 to 5,000 and having an acid number of 30 to 150, and C) 10 to 50% by weight of ethylenically unsaturated monomers, 10 to 50% by weight of the monomers containing carboxyl groups, where the sum of A), B) and C) is 100% by weight, the binder a) has an acid number of 20 to 150, and the crosslinking agents b) used are phenolic and/or amino resins, with the proviso that the coating agent contains a) 30 to 70% by weight of the binder a), b) 2 to 30% by weight, preferably 5 to 16% by weight, of the crosslinking agent b), c) 1 to 7% by weight, preferably 2 to 5% by weight, of ammonia and/or amine as neutralizing agent, and d) 20 to 60% by weight of organic solvents, where the sum of a), b), c) and d) is 100% by weight. The present invention also relates to a process for the preparation of the coating agents, and the use thereof for coating of cans.

We claim: 
     
       1. A process for the preparation of an aqueous coating agent wherein the coating agent comprises: (a) 30 to 70% by weight of a binder having an acid number of 20 to 150 and comprised of the reaction product of: (A) 20 to 80% by weight of an epoxy resin having an average of more than one epoxy group per molecule and having an average molecular weight of at least 500,   (B) 1 to 60% by weight of a polyester polycarboxylic acid having an average molecular weight of 500 to 5,000 and having an acid number of 30 to 150, and   (C) 10 to 50% by weight of ethylenically unsaturated monomers, 10 to 50% of which contain carboxy groups, and a peroxide initiator which is present in an amount which is at least 2% by weight relative to the weight of the ethylenically unsaturated monomers,   the sum of (A), (B), and (C) being 100% weight;     (b) 2 to 30% by weight of a phenolic or amino resin crosslinking agent;   (c) 1 to 7% by weight of ammonia or an amine neutralizing agent, and   (d) 20 to 60% by weight of an organic solvent, the sum of a), b), c) and d) being 100% by weight; the process comprising the steps of:     
     
     
       1. reacting the epoxy resin A) with the polyester polycarboxylic acid component B) at a temperature of about 80° to 200° C. in the presence of catalyst so that at least 80% of the oxirane rings initially present are opened; 2.   
     
     
       2. polymerizing component C) in the presence of the reaction product of step 1 at a temperature of about 100° to 160° C.; 3. neutralizing the polymerization product of step 2 using component (c);   4. adding the organic solvent d) and the crosslinking agent b) to the neutralized product of step 3 to form the coating agent, and   
     
     
       5. dispersing the coating agent in water. 2. A process as claimed in claim 1, wherein the peroxide initiator is at least 2.6% by weight relative to the total weight of the ethylenically unsaturated monomers. 
     
     
       3. A process as claimed in claim 1, wherein the peroxide initiator is at least 3.0% by weight relative to the total weight of the ethylenically unsaturated monomers. 
     
     
       4. A process as claimed in claim 1, wherein the epoxy resin A) is based on bisphenol A. 
     
     
       5. A process as claimed in claim 1 wherein the epoxy resin A) has an average molecular weight of at least 3,000. 
     
     
       6. A process as claimed in claim 1, wherein the polyester polycarboxylic acid B) has an average molecular weight of 1,000 to 3,000 and an acid number of 50 to 100. 
     
     
       7. A process as claimed in claim 1, wherein the polyester polycarboxylic acid contains an alcohol component which is an ester diol and/or a glycidyl ester of a monocarboxylic acid. 
     
     
       8. A process as claimed in claim 1, wherein the ethylenically unsaturated monomers C) comprise x) 10 to 50% by weight of a monomer containing carboxyl groups,   y) 0 to 50% by weight of a nonfunctionalized monomer, and   z) 5 to 60% by weight of a (meth)acrylate having 1 to 20 carbon atoms in the alcohol radical   where the sum of x), y) and z) is 100% by weight.   
     
     
       9. A process as claimed in claim 1, wherein the ethylenically unsaturated monomers C) comprise x) 20 to 40% by weight of a monomer containing carboxyl groups,   y) 20 to 40% by weight of a nonfunctionalized monomer, and   z) 10 to 50% by weight of a (meth)acrylate having 1 to 20 carbon atoms in the alcohol radical   where the sum of x), y) and z) is 100% by weight.   
     
     
       10. A process as claimed in claim 1 wherein the binder a) is comprised of from 35 to 60% by weight of A), 10 to 35% by weight of B) and 15 to 30% by weight of C). 
     
     
       11. A process as claimed in claim 1 wherein b) is a phenolic resin resol based on bisphenol A which contains more than one methylol group per phenyl ring. 
     
     
       12. A process as claimed in claim 1 wherein the neutralizing agent c) is triethylamine or dimethylethanolamine. 
     
     
       13. The process as claimed in claim 1, wherein the polymerization product from step 2 is precondensed with the crosslinking agent b) prior to the neutralization step 3. 
     
     
       14. The process as claimed in claim 1, wherein less than 0.2% by weight, relative to the sum of components a) to d), of residual monomers are present after the polymerization step 2. 
     
     
       15. The process as claimed in claim 1, wherein the peroxide initiator is dibenzoyl peroxide and/or tert. butyl perbenzoate. 
     
     
       16. The process as claimed in claim 1, wherein component d) is used a solvent in the esterification process of step 1. 
     
     
       17. The process of claim 1, wherein the reaction of epoxy resin A) and the polyester polycarboxylic acid (B) in step 1 is at a temperature of about 120° to 180° C. 
     
     
       18. The process of claim 1, wherein the polymerization of component C) in step 2 is at a temperature of about 120° to 140° C. 
     
     
       19. The process of claim 1, wherein the peroxidic initiator generates benzoyloxy or phenyl free radicals. 
     
     
       20. The process of claim 1, wherein the coating agent comprises 5 to 16% by weight of component (b). 
     
     
       21. The process of claim 1, wherein the coating agent comprises 2 to 5% by weight of component (c).